Platelet production is now known to be controlled by way of a humoral mechanism much like erythropoiesis, but further research is hampered because of the lack of adequate assay methods. This research project is designed to develop both a bioassay and an immunoassay for thrombopoietin, and to purify and identify the hormone, thrombopoietin. These objectives will be pursued by utilizing a variety of physical, chemical and immunological methods: anti-platelet serum and irradiation will be used to produce thrombopoietin-rich serum; thrombopoietin will be partially purified by several chemical methods; a bioassay procedure for thrombopoietin will be developed; anti-thrombopoietin serum will be raised in rabbits; and further development of an immunoassay for thrombopoietin by use of the hemagglutination-inhibition system will be performed. The development of a reproducible quanitative assay technique for thrombopoietin will make possible studies of thrombopoietin activity in the serum of patients with platelet production disorders.